Electron discharge device



y 1960 M. CHODOROW 2,937,311

ELECTRON DISCHARGE DEVICE Original Filed Oct. 12, 1953 FIG 1 FIE ammvroa. IE I I5 a FIEIL l Marvin Chodorow Attorney United States PatentELECTRON DISCHARGEiDEVICE Marvin Chodorow, Menlo.Park, Calif.,..assignon,td -Vai'iail tssociates, San Carlos, Califqa corporationof CahormaOriginal applicationOctoberlZ, 1953, Serial No.'385, 357,

now Patent No. 2,836,758,-dated May 27,1958, Divided and thisapplication November 13, 1957,; Serial 5 Claims. *(Cl. 31 5'3.6)

' The present invention relatesito electron dischargefdevices, such astraveling wave tubes whereinienergy'isexchanged between an electronbeam'andthefieldof a traveling radio frequency wave,an'd more particularlyto -the structure which carries the radio 'frequency' wave within suchdevices.

' In many traveling wave tubes, the 'radio'frequency wave ispropagatedon a helical structure, 'whoseidiameter and pitch are suchthatthelongitudinalphase velocity of the wave is substantially equal tothe"velocity of the electron beam' which is'directed longitudinally ofthe helix. To insure optimum energy exchange'or interaction between theelectron beam and'the. radio frequency field, the operating voltageaswell as :thegeometry of the helix must be properly chosen. It has beendetermined that good interactionis obtainable in tice by meeting thecondition pracvision of a very small electron beam-diameter which isimpractical to achieve. Thus the stated condition, in

eifect, represents the practical operating range'of a traveling Wavetube employing. a single helix asv a slow wave structure.

Theforegoing condition may be met quite readily when a traveling wavetube is operated withalow voltage so that good interaction andconsequently a favorable gain characteristic may be attained, butcertain difiiculties are encountered in tubes employing ,a single helixslow wave structure when operated at relatively vhigh voltages kv. orgreater).

As the voltage is raised, .the beam velocity increases. Consequently, itis desirable that the radius, a, of the helix be increased acorresponding amount to enable operation in the' range established bythe described'condition. Tocorrelate the lQngitudinalphase velocity ofthe radio frequency wave with the increased velocity of the electronbeam, it is also necessary to' increase the pitchof the helix. It hasbeenfound that such an increase in the radius and the pitch of a helixreduces its impedance and ultimately the gain characteristic of thewhere:

-E=interaction field component or longitudinal afield component of thetraveling wave,

. ,B= -phase constant of the wave and,

P-- total power flow.

-When the helix becomes larger inradiusan'd' ingpitch, the otherFouriercomponents of the wave, -which travel forwardly at difierent phasevelocities 'thanthe -inter- 10- action componennE, and do not interactwith, theelectron beam, carry, relatively speaking a larger amount orthe total power, P, thereby subtracting from *the en- .ergy in thelongitudinally directed'interaction "compo nent, E, of the wave, to thusreduce'the impedance value. An additional difiiculty-encountered in theoperation of -a traveling wave tube embodyinga single helix'as a slowwave structure-concerns the production of oscillations by way ofthe-so-called backwardwave? *These backward wave oscillations i resultfrom A the interaction of the electron beam with a Fouriercomponent-ofthe wave -in-which theenergy is propagating inthe' oppositeoptimize the energy content of the interaction component of'thetraveling wave.

direction to the beam. This Fourier component has a phase velocityin-the direction-of the beam -and'substantially equal to its velocity.Such oscillations ;are

possible in any traveling-wave tube embodyingafsingle helix as a.slowwave structure andparticu-larly"if the beam almost fills the helix.These-oscillations are undesiredin an amplifier tube.

videa'slowwave'structure for a traveling wavetu-be or similar electronicdischarge device which -w-i-llprovide optimumgain characteristicsforhigh voltage operation. Mor-eparticularly,- it is an object to a providea-- slow wave structure for a traveling wave tube whiehwill A furtherobject --isto i provide a slow wave structure whichreduces the power inthose field components of the traveling wave which do not interact withtheelectron beam.

Another object" of the -invention-is to provide a;.-slow -wave structurewhich-substantially reduces theproducition of backward waveoscillations.

Still another object of the present inventionis to provide a novel slowwave structure whichembodies a series ofspaced, axially-aligned parallelrings, each of..which is joined at-diametrically-opposed-pointsrespectively to the rings on eachside thereof.

These and other, objects -as well -as the advantages arising from the"presentinvention will be apparent-from the following description ofapreferred embodiment thereof as shown inthe drawings wherein: Y

Fig. 1 is a sectional view, in part diagrammatic of a travelingwave tubeembodying the present invention,

i Fig. 2- is a cross-sectional view taken along line 2'Z in Fig 1,

Fig. 3 'isafragmentary-view of the slow wave structure shown inFigxl asseen from the top thereof,

Fig. 4 is a section taken along line-4-4 of Fig. 3,

Fig. 5 :isa fragmentary side view of the slow-wave structure of Fig. 1,

. slow wave structure embodyingthe present invention.

longitudinally of the tube.

Generally, the present invention is embodied in a slow wave structurewhich reduces the undesired Fourier comin effect, by winding two helicesof substantially the same diameter in opposite directions and applyingthe radio frequency wave to the resulting slow wave structure in amanner such that the -varying components are reduced and the useful beaminteraction comp-onents of the wave propagated on the two contra-woundhelices become additive. Since the -varying components encountered inthe use of a single helix are reduced by the described slow wavestructure, high voltage operation of a traveling wave tube will bepermitted with relatively good gain characteristics and backwardwaveoscillations will be substantially eliminated.

The two modes on oppositely wound helices which correspond to theordinarily used mode of a single helix can be of the same or of oppositepolarity. The first mode, where the condition of opposite polarityexists, produces a field between the two helices of relatively greatstrength and an axial or longitudinal field of negligible value.voltages are of the same polarity, is, to the contrary, possessed of asmall field between the helices, and a large longitudinal component,which is that useful for interaction with the beam. Accordingly, it canbe seen that if radio frequency energy is applied to contra-woundhelices constructed in accordance with the present invention so that theproper polarity condition is fulfilled, the first mode mentioned abovewill in eifect be eliminated and only the useful second mode will appearon the slow wave structure. helix voltages are of like polarity, it isnot necessary that the helices be insulated one from the other.

In Fig. 1 is shown one embodiment of the invention in a traveling wavetube which includes a cylindrical glass casing closed at one end by acollector 11. The collector 11 is hermetically sealed at its peripheryto the glass casing 10 and has a central bore 12 for the collection ofelectrons which have traversed the length of the traveling wave tube.The collector 11 is maintained at a desired positive potentialwithrespect to the cathode 13 of the tube by a suitable direct currentvoltage source indicated at 14; the cathode 13 beingmounted at the otherextremity of the glass casing 10 within a glass cup 15 which is joinedthereto in sealed relation by the cylindrical peripheral portion 16a ofa focusing ring 16. The juncture is such that the focusing ring 16 issupported so as to direct electrons emitted from the cathode 13 insubstantially a small, rectilinear beam axially or The cathode 13 isheated by a suitable filament connected to a filament voltage sourceshown schematically at 17 to produce the described emission and apositive potential is applied to the focusing ring 16 from a directcurrent voltage source 19. The electrons are maintained in 'a small beamthrough the tube by means of a suitable magnetic focus-- ing coil 18. r

The slowwave structure, generally indicated by the numeral 20, issupported axially of the tube between the cathode and collector endsthereof by means of three glass rods 21 (Fig. 2) and is provided at eachend with an antenna 22 which preferably takes the form of a hollowcylindrical stub. The antenna 22 at the cathode end of the tube receivesradio frequency energy from an energy translating device such as aconventional waveguide 23 which is tapered as it approaches the tube toinsure proper matching with the slow wave structure and is additionallyprovided with a short 'lateral flange 24 adjacent the antenna 22 toincrease the coupling therebetween. A like structural arrangementprovides for coupling radio frequency energyfrom the antenna 22 at thecollector end of the tube into an output waveguide section 25. Positivedirect current voltage is sup- The second mode, where the two helixSince this second mode is that where the 4 plied to the slow wavestructure 20 by connection to the collector 11, as indicated at 20 Itwill be apparent that the radio frequency energy can be coupled to therespective ends of the slow wave structure 20 by conventional coaxiallines.

The center conductor of each coaxial line is connected to one end of theslow wave structure at the antenna terminal thereof and the outerconductor can be connected to a metallic shield surrounding the glasscasing,

The slow wave structure 20 shown in Figs. 1, 2, 3, 4 and 5 embodies thecontra-wound helices previously discussed, the structure beingfabricated in a particularly simple manner. A series of V-shapedjnotches26 are cut along one side of a cylindrical metal tube 27 by a millingcutter, indicated in phantom at 28 in Fig. 3, and which passes throughthetube as shown by the arrow A in Fig. 4. The tube is then rotatedone-half turn and a second series of notches '29 staggered with respectto the first series but of the same configuration are cut from this sideof the tube 27.

As shown in Fig. l, the notches 26, 29 are enlarged adjacent the inputand output ends to increase the pitch of the helices and thus establishproper matching with the input and output waveguide circuits.Additionally, to insure that the previously described requirement oflike polarity of the voltages on the contra-Wound helices be met, eachantenna is connected to the helices at a common antenna terminalindicated at 30. Thus two helical and electrically parallel .paths forradio frequency energy flowing through the opposed elements of thehelices are provided by the slow Wave structure of Fig. 1, which, aspreviously discussed, both reduce the power lost in the non-interactingFourier components and substantially eliminate backward waveoscillations.

Alternatively, other structures can be fabricated which, in efiect,provide helical and electrically parallel paths for a radio frequencywave. As shown in Fig. 6, a slow'wave structure embodying thecontra-wound helix principle comprises by a series of axially-alignedparallel rings 31 each of which is joined to the rings on oppositesides'thereof at points spaced with short longitudinally extendingconducting bars 32 whose length is, of course, determined by thevelocity of the beam and the frequency of the radio frequency wave. Sucha structure is conveniently formed, for example, by means of notchessuitably placed along a metal tube, as shown in Fig. 6. Both this andthe first described slow wave structure are mechanically rigid-so thatthey can be mounted in various manners within a tube depending uponspace and structural limitations.

In Figs. 7 and 8 is shown a modification providing helices insulated onefrom the other and which includes a cylindrical glass tubular support 33having helices 34, 35 formed of wire applied to the interior andexterior surfaces thereof. The only limitation on such a structure isthat the thickness of the tubular support33 be not too great so that thediameters of thehelices 34, 35 will not be too variant. Otherwise, acertain amount of power-will be carried by the'qs-varying components andwill thus be lost as useful energy.

The contrawound helices can, ofcourse, be formed of wires 36, 37.both ofwhich are wound on the exterior of a glass tube 38, as shown in Fig. 9.Since, in every case, the helices are arranged so as to propagate themode where the voltages are of like polarity, it is irrelevant, asmentioned hereinbefore whether the helices contact as here in Fig. 9 orare insulated from one another as shown in Figs. 7 and 8.

It will be observed that each of the various described contra-woundhelix structures is supported by a glass cylinder or by glass tubes.Such supports, when utilized with a single helix slow wave structure,cause a reduction in the impedance as previously defined. However, thenoted reduction of power carried by the non-interacting Fouriercomponents of the radio frequency wave, which results from thecontra-wound helix construction, substantially reduces the lossesincurred in glass or any other dielectric supporting members.Consequently, from a number of aspects, the contra-wound helix slow wavestructure embodying the present invention optimizes the exchange ofenergy between an electron beam and the field of a traveling radiofrequency wave.

Various other modified versions of the slow wave structure embodying thepresent invention are obviously possible. described, may be employed toadvantage not only in traveling wave tubes, but also in other electrondischarge devices wherein an electron beam is to exchange energy with aradio frequency field; one example being the linear accelerator. In viewof these alternatives and possible further modifications, I do not wishto be limited to the specific structure recited hereinabove. Rather,such structure should be considered merely as exemplary, and theintended scope of the invention is given by reference to the appendedclaims.

What is claimed is:

1. In an electron discharge device wherein energy is exchanged betweenan electron beam and the field of a Additionally, the slow wavestructure, as herein radio frequency wave, apparatus for propagating theradio frequency wave comprising a series of spaced axiallyalignedparallel rings each of which is joined at diametrically opposite pointsrespectively to the rings on each side thereof, the halves of said ringsdefined by said points providing two electrically parallel paths for thepropagation of the wave energy.

2. In an electron discharge device as defined in claim 1 whereinconducting bars are used to join diametrically opposite points of eachof said rings respectively to the rings at each side thereof, an energytranslating device for interchanging energy with said propagatingapparatus, said slow wave structure having a single conducting barconnected to an end ring thereof and coupled to said translating devicesuch that the currents in the halves of said rings are in phase, equaland in parallel, whereby longitudinal field components of the wave areadditive along such structure.

3. In an electron discharge device wherein energy is exchanged betweenan electron beam and the field of a radio frequency wave, apparatus forpropagating the radio frequency wave comprising a slow wave structureprovided with a series of periodically spaced conductive rings ofuniform diameter alternately connected together with longitudinallyextending conductors at diametrically opposite points on the rings, andmeans for setting up an electron beam and for passing the same throughsaid propagating apparatus.

4. An electron discharge device as defined in claim 3 wherein a pair ofenergy translating devices are provided for interchanging energy withthe ends of said propagating apparatus, said slow wave structure havingsingle antenna terminals connected to bothend rings thereof, saidantenna terminals respectively coupling the ends of said slow wavestructure to a respective one of said translating devices.

5 In an electron discharge device wherein energy is exchanged between anelectron beam and the field of a .radio frequency wave, the combinationcomprising: a

slow wave structure for propagating the radio frequency wave comprisinga series of spaced axially-aligned parallel rings each of which isjoined at diametrically opposite points respectively to the rings oneach side thereof and an antenna terminal at each end of said structurediametrically opposite the point at which the end ring is joined to thering adjacent thereto; a pair of energy translating devices forinterchanging energy with the ends of said slow wave structure, saidantenna terminals respectively coupling the ends of said slow wavestructure to a respective one of said translating devices; and means forsetting up an electron beam and forpassing the sam through said slowwave structure.

References Cited in the file of this patent UNITED STATES PATENTS2,725,499 Field Nov. 29, 1955 2,768,322 Fletcher Oct. 23, 1956 2,798,183Sensiper July 2, 1957 2,813,221 Peter Nov. 12, 1957 2,822,502 SensiperFeb. 4, 1958 2,824,257 Branche Feb. 18, 1958 I FOREIGN PATENTS 668,017Great Britain Mar. 12, 1952

